The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is one of the most devastating corn rootworm species in North America and is a particular concern in corn-growing areas of the Midwestern United States. The northern corn rootworm (NCR), Diabrotica barberi Smith and Lawrence, is a closely-related species that co-inhabits much of the same range as WCR. There are several other related subspecies of Diabrotica that are significant pests in North America: the Mexican corn rootworm (MCR), D. virgifera zeae Krysan and Smith; the southern corn rootworm (SCR), D. undecimpunctata howardi Barber; D. balteata LeConte; D. undecimpunctata tenella; and D. u. undecimpunctata Mannerheim. The United States Department of Agriculture currently estimates that corn rootworms cause $1 billion in lost revenue each year, including $800 million in yield loss and $200 million in treatment costs.
Both WCR and NCR are deposited in the soil as eggs during the summer. The insects remain in the egg stage throughout the winter. The eggs are oblong, white, and less than 0.004 inch (0.010 cm) in length. The larvae hatch in late May or early June, with the precise timing of egg hatching varying from year to year due to temperature differences and location. The newly hatched larvae are white worms that are less than 0.125 inch (0.318 cm) in length. Once hatched, the larvae begin to feed on corn roots. Corn rootworms go through three larval instars. After feeding for several weeks, the larvae molt into the pupal stage. They pupate in the soil, and then they emerge from the soil as adults in July and August. Adult rootworms are about 0.25 inch (0.635 cm) in length.
Corn rootworm larvae complete development on corn and several other species of grasses. Larvae reared on yellow foxtail emerge later and have a smaller head capsule size as adults than larvae reared on corn. Ellsbury et al. (2005) Environ. Entomol. 34:627-34. WCR adults feed on corn silk, pollen, and kernels on exposed ear tips. If WCR adults emerge before corn reproductive tissues are present, they may feed on leaf tissue, thereby slowing plant growth and occasionally killing the host plant. However, the adults will quickly shift to preferred silks and pollen when they become available. NCR adults also feed on reproductive tissues of the corn plant, but in contrast rarely feed on corn leaves.
Most of the rootworm damage in corn is caused by larval feeding. Newly hatched rootworms initially feed on fine corn root hairs and burrow into root tips. As the larvae grow larger, they feed on and burrow into primary roots. When corn rootworms are abundant, larval feeding often results in the pruning of roots all the way to the base of the corn stalk. Severe root injury interferes with the roots' ability to transport water and nutrients into the plant, reduces plant growth, and results in reduced grain production, thereby often drastically reducing overall yield. Severe root injury also often results in lodging of corn plants, which makes harvest more difficult and further decreases yield. Furthermore, feeding by adults on the corn reproductive tissues can result in pruning of silks at the ear tip. If this “silk clipping” is severe enough during pollen shed, pollination may be disrupted.
Control of corn rootworms may be attempted by crop rotation, chemical insecticides, biopesticides (e.g., the spore-forming gram-positive bacterium, Bacillus thuringiensis), or a combination thereof. Crop rotation suffers from the significant disadvantage of placing unwanted restrictions upon the use of farmland. Moreover, oviposition of some rootworm species may occur in soybean fields, thereby mitigating the effectiveness of crop rotation practiced with corn and soybean.
Chemical insecticides are the most heavily relied upon strategy for achieving corn rootworm control. Chemical insecticide use, though, is an imperfect corn rootworm control strategy; over $1 billion may be lost in the United States each year due to corn rootworm when the costs of the chemical insecticides are added to the costs of the rootworm damage that may occur despite the use of the insecticides. High populations of larvae, heavy rains, and improper application of the insecticide(s) may all result in inadequate corn rootworm control. Furthermore, the continual use of insecticides may select for insecticide-resistant rootworm strains, as well as raise significant environmental concerns due to the toxicity of many of them to non-target species.
RNA interference (RNAi) is a process utilizing endogenous cellular pathways, whereby an interfering RNA (iRNA) molecule (e.g., a dsRNA molecule) that is specific for all, or any portion of adequate size, of a target gene sequence results in the degradation of the mRNA encoded thereby. In recent years, RNAi has been used to perform gene “knockdown” in a number of species and experimental systems; for example, Caenorhabitis elegans, plants, insect embryos, and cells in tissue culture. See, e.g., Fire et al. (1998) Nature 391:806-11; Martinez et al. (2002) Cell 110:563-74; McManus and Sharp (2002) Nature Rev. Genetics 3:737-47.
RNAi accomplishes degradation of mRNA through an endogenous pathway including the DICER protein complex. DICER cleaves long dsRNA molecules into short fragments of approximately 20 nucleotides, termed small interfering RNA (siRNA). The siRNA is unwound into two single-stranded RNAs: the passenger strand and the guide strand. The passenger strand is degraded, and the guide strand is incorporated into the RNA-induced silencing complex (RISC). Micro inhibitory ribonucleic acid (miRNA) molecules may be similarly incorporated into RISC. Post-transcriptional gene silencing occurs when the guide strand binds specifically to a complementary sequence of an mRNA molecule and induces cleavage by Argonaute, the catalytic component of the RISC complex. This process is known to spread systemically throughout the organism despite initially limited concentrations of siRNA and/or miRNA in some eukaryotes such as plants, nematodes, and some insects.
Only transcripts complementary to the siRNA and/or miRNA are cleaved and degraded, and thus the knock-down of mRNA expression is sequence-specific. In plants, several functional groups of DICER genes exist. The gene silencing effect of RNAi persists for days and, under experimental conditions, can lead to a decline in abundance of the targeted transcript of 90% or more, with consequent reduction in levels of the corresponding protein.
U.S. Pat. No. 7,612,194, and U.S. Patent Publication Nos. 2007/0050860 and 2010/0192265, disclose a library of 9112 expressed sequence tag (EST) sequences isolated from D. v. virgifera LeConte pupae. It is suggested in U.S. Pat. No. 7,612,194 and U.S. Patent Publication No. 2007/0050860 to operably link to a promoter a nucleic acid molecule that is complementary to one of several particular partial sequences of D. v. virgifera vacuolar-type H+-ATPase (V-ATPase) disclosed therein for the expression of anti-sense RNA in plant cells. U.S. Patent Publication No. 2010/0192265 suggests operably linking a promoter to a nucleic acid molecule that is complementary to a particular partial sequence of a D. v. virgifera gene of unknown and undisclosed function (the partial sequence is stated to be 58% identical to C56C10.3 gene product in C. elegans) for the expression of anti-sense RNA in plant cells.
No further suggestion is provided in these publications to use any particular sequence of the more than nine thousand sequences listed therein for RNA interference, other than the several particular partial sequences of V-ATPase and the particular partial sequence of a gene of unknown function. Furthermore, none of U.S. Pat. No. 7,612,194, and U.S. Patent Publication Nos. 2007/0050860 and 2010/0192265 provides any guidance as to which other of the over nine thousand sequences provided would be lethal, or even otherwise useful, in species of corn rootworm when used as dsRNA or siRNA. The overwhelming majority of these sequences are not lethal in species of corn rootworm when used as dsRNA or siRNA. For example, Baum et al. (2007) Nat. Biotech. 25(11):1322-6, describe the effects of inhibiting several WCR gene targets by RNAi. These authors reported that 8 of 26 target genes they tested were not able to provide experimentally significant coleopteran pest mortality at a very high iRNA (e.g., dsRNA) concentration of more than 520 ng/cm2.